Medical imaging is accomplished by inducing a patient with a radiopharmaceutical substance by injection swallowing, inhalation, or other appropriate means. The radioactive isotope of the radiopharmaceutical selectively migrates to the target area tissue to be examined and emits gamma radiation from it. Reference herein to “radiation” is to any ionizing radiation, but typically gamma radiation. The radiation can be sensed and used to generate a reconstructed image of features of the target area tissue to provide diagnostic information for appropriate treatment.
The sensing of radiation from target tissue area is typically accomplished by means of a gamma camera. Such a camera features a detector head including a round or rectangular camera plate optically coupled to a corresponding two-dimensional array of position-sensitive photosensors, typically photomulitplier tubes. The array of photosensors may have a view of the camera plate which is about 30 (centimeters) or more in its major dimension. Detector heads weighing hundreds of pounds are used to make two-dimensional images, sometimes in a stationary mode and sometimes in a scanning mode. They can also be used to make three dimensional images by taking a plurality of views of the same target from different angles and using computer logic image reconstruction techniques.
A so called “gamma camera plate” is a large area device for converting radiation to light and is most commonly an assembly of a scintillation crystal slab, such as sodium iodide doped with thallium for activation, which is hermetically sealed in a housing. The housing is made up of a shallow aluminum pan “back cap” covered with a glass optical window bonded to the back cap about its perimeter. An optical interface is provided between the crystal and the window to improve the coupling. Such an assembly is described, for example, in U.S. Pat. No. 5,874,738 issued Feb. 23, 1999 to Scott R. Huth and assigned to the same assignee as is the present invention.
In operation, radiation from the target enters the crystal from the back cap radiation entrance side of the camera plate. The radiation interacts with the crystal to result in light scintillation inside it. The light passes out of the plate through the optical window and into an array of photomultipliers which are coupled to its outside surface to convert the light to electrical signals. The electrical signals are fed to a digital processor for the construction of image information in a graphic form. The processor software has the capability for accounting to some extent for spreading of the light inside the crystal between the point of its creation and its exit from the window into the photomultipliers. The spreading results in some loss of reconstructed image resolution and is undesirable in that respect, but it is at the same time also necessary to some extent for determination of position information by comparing the signal response of several nearby photomultiplier tubes to the same scintillation event.
Medical imaging may variously require low or high energy radiation, depending on the type and thickness of tissue being examined. Common low energies for medical imaging are those up to 140 keV (thousand electron volts), such as 80 keV available from the isotope thallium-201 and 140 keV available from the isotope technetium-99m. A common high energy is, for example, 511 keV, such as is available from the isotope fluorine-18. The radiation's mean depth of interaction with the crystal is close to the radiation entrance side for low energies, but deeper for the higher energies. This presents a serious tradeoff problem with regard to the choice of thickness for the crystal. A crystal which is the optimum thickness for low energy radiation is too thin to effectively capture high energy radiation. On the other hand, a crystal with an optimum thickness for high energy radiation suffers from excessive light spreading with low energy radiation, thereby reducing the image resolution. Therefore, in order for an imaging system to be usable with good resolution for both low and high energy radiation, it would be necessary to change the gamma camera plate. However, this is a costly component which is also closely matched to the associated electrical hardware and software and therefore not easily exchanged for one with different characteristics. There is a need, therefore, for a gamma camera plate which can be satisfactorily used with both low and high energy radiation.